As is well known, a printed circuit board (PCB) is a thin laminated board having multiple integrated circuit (IC) chips mounted thereon. Multiple conductive paths are formed on the PCB to provide communication among the multiple IC chips. PCBs commonly have an edge connector provided along one side which is adapted to be inserted into a receptacle on a control board, such as a "mother board" in a personal computer. The edge connector has conductive leads, formed of solder or other conductive material, which contact connectors provided in the receptacle. As a result, the PCB is electrically coupled to the mother board.
The thickness of the circuit boards, and particularly the thickness of the edge connectors, is very important to ensure proper electrical contact. The connectors housed in the receptacle are often shaped in a clamp-like manner to grip the edge connector when the PCB is inserted into the receptacle. The clamp-like connectors can scrape a significant amount of the conductive solder away from the edge connectors if the edge connectors are too thick, resulting in a poor fit and poor electrical contact. On the other hand, if the edge connectors are too thin, a loose fit results, causing poor or intermittent electrical contact.
Therefore, many companies implement a testing process to measure the thickness of the edge connectors of PCBs prior to shipment or insertion into receptacles on a control board. Such tests assist in controlling the quality of the PCB supplied to distributors and ultimate end users by ensuring that only boards with accurate thickness are shipped and installed.
One technique for measuring the thickness of PCBs involves a mechanical sensor which is first initialized to a flat reference surface, and then maneuvered over the edge of the PCB as the PCB lies on the reference surface. A distance signal is generated based upon the sensor location in relation to the reference surface. The mechanical sensor employs a small roller which is rolled over the edge connector area of the PCB.
The mechanical sensor follows the surface of the PCB, which is not perfectly flat. The edge connector of the PCB typically has multiple conductive leads spaced evenly along the edge. The conductive leads are formed on the board through a process which leaves the conductive leads slightly raised from the board. Accordingly, as the mechanical sensor is moved across the edge connector of the PCB, the mechanical sensor moves up and down over the conductive leads. An average thickness is then computed based on the thickness profile measured across the entire length of the edge connector.
Unfortunately, the above method is labor intensive. Each PCB must be measured by hand. Thus, the measuring system is impractical for high volume production.
One proposed technique for measuring the thickness of a PCB employs a laser. A laser is aimed at the PCB which is resting on a flat reference surface, and the light reflected from the PCB is monitored. Analysis is then conducted on the reflected light to determine a thickness of the PCB. The laser technique works effectively on flat surfaces. The laser technique does not, however, work effectively on non-flat, non-smooth surfaces because the light reflected from uneven surfaces is skewed which introduces significant error into the thickness analysis. Thus, an accurate reading of the entire thickness of a PCB board, including the core and the conductive leads, cannot be accurately obtained.
A method according to the present invention provides a new approach to measuring the thickness of the PCB edge connector. The method accurately measures flat and non-flat surfaces of a PCB.